The invention relates to the field of watermarking of audio and video signals.
The invention relates to a method and arrangement for embedding supplemental data in an information signal. The information signal is encoded by an encoder including a feedback loop. Selected samples of the encoded signal are modified within the feedback loop to represent the supplemental data and synchronization bit pattern. The modified samples representing the supplemental data are spaced apart by at least a first number of samples.
There is a growing need to accommodate watermarks in audio and video signals. Watermarks are supplemental data messages embedded in multimedia assets, preferably in a perceptually invisible manner. They comprise information, for example, about the source or copyright status of documents and audiovisual programs. They may be used to provide legal proof of the copyright ownership, allow tracing of piracy and support the protection of intellectual property.
A known method of embedding supplemental data in an information signal as described above is disclosed in International Patent Application WO-A-98/33324. In this known method, a watermark pattern is embedded in a (sigma-)delta-modulated audio signal. Each bit of such a unit-bit coded signal is a signal sample. The watermark is embedded in the encoded audio signal by modifying selected bits thereof. For example, every 100th bit is replaced by a bit of the watermark pattern. The step of modifying the encoded audio signal is carried out inside the feedback loop of the encoder so as to compensate the effect of the modification in subsequent encoding steps.
The known method is envisaged for recording high-quality audio on the audio version of the Digital Versatile Disk (DVD). A sampling frequency of 2,822,400 Hz (64*44,100) will be used to yield a signal-to-noise ratio of 115 dB. Replacing every 100th bit of the sigma-delta-modulated audio signal by a watermark bit at the expense of only 1 dB increases the quantization noise. This corresponds to a watermark bit rate of about 28000 bits per second.
The above-mentioned patent application WO-A-98/33324 also discloses an arrangement for extracting the watermark. The arrangement includes a divider stage and a sync detector. The divider stage divides the bit rate by the number of bits by which the watermark bits are spaced apart (e.g. 100 if every 100th bit of the signal is a supplemental data bit). The sync detector changes the phase of the divider stage until a synchronization bit pattern (hereinafter sync pattern for short) is detected in the bit stream.
It will be appreciated that the sync detector necessarily includes a shift register (or serial-to-parallel converter) to store a portion of the bit stream. In the known method, the sync pattern is accommodated in the watermark, i.e. the sync pattern bits are spaced apart by the same number of bits as the watermark bits. This requires a long shift register in practice. The length of the shift register depends on the length of the sync pattern and the distance between the watermark bits. If every Mth bit of the signal is a supplemental data bit and the sync pattern comprises N bits, the sync detector must necessarily store (Nxe2x88x921)xc2x7M+1 bits.
German Patent Application DE-A-37 17 315 discloses such a known sync detector in more details. In this publication, every 15th bit of a signal is a supplemental bit and the sync pattern is a 4-bit word. In accordance therewith, the serial-to-parallel converter (reference numeral 5 in FIG. 2 of DE-A-37 17 315) holds 46 bits.
The above citations are hereby incorporated whole by reference.
It is an object of the invention to provide a method of embedding supplemental data in an information signal, which allows the supplemental data to be extracted in a more cost-effective manner.
To this end, in the method in accordance with the invention the step of modifying includes spacing apart the modified samples representing the synchronization bit pattern at most by a second number of samples which is substantially smaller than the first number of samples.
The length of the shift register in the sync detector is now determined by the sync pattern length and the second number of bits. The second number can be chosen to be independent of the first number and may be arbitrarily small or even zero. In the latter case, the sync pattern bits are successive bits of the encoded signal. The length of the shift register then corresponds to the length of the sync pattern.
In a preferred embodiment of the invention, the synchronization bit pattern is a bit pattern which is typically not generated by the encoder. The sigma-delta modulator, for example, which is envisaged for recording high-quality audio on DVD, produces a bit stream with a high-frequency pattern of zeroes and ones. The modulator tries to alternate the output bits as fast as possible so as to move quantization errors out of the audio band. Typically, the sigma-delta modulator does not produce a large number of ones followed by a large number of zeroes. For example, the bit pattern 11110000 has not been found in recordings of music. Forcing the modulator to generate such an atypical or non-characteristic pattern within the feedback loop causes the modulator to rapidly change the bit stream to the high-frequency pattern mentioned above. Such an atypical pattern is an excellent candidate for constituting the sync pattern.